SLAA486B January   2011  – June 2019 MSP430G2452

 

  1.   EKG-Based Heart-Rate Monitor Implementation Using the MSP430G2xx LaunchPad™ Development Kit
    1.     Trademarks
    2. 1 Introduction
    3. 2 Hardware Description
      1. 2.1 Hand Detection Circuit
      2. 2.2 Analog Front End (AFE)
      3. 2.3 Pin Functionality Mapping on the LaunchPad Kit
      4. 2.4 eZ430 RF Target Header
      5. 2.5 Power Supply Setup
    4. 3 Software
    5. 4 References
  2.   A Amplifier Options
    1.     A.1 Instrumentation Amplifier Options
    2.     A.2 Operational Amplifier Options
  3.   B Wired USB Demo With Back-Channel UART
  4.   C Wireless UART Demo With the eZ430 RF Target Board
  5.   D Hardware Schematic Diagrams
  6.   Revision History

2.2 Analog Front End (AFE)

As mentioned previously, the amplitude of the EKG signal is approximately 1 mV peak-to-peak. The noise signals picked up by the human body (such as the 50- to 60-Hz line frequency) pose a serious problem to detecting the low-frequency low-magnitude EKG signal. An analog front end with a high gain with low cutoff filter frequency is necessary to condition this signal for digital conversion and processing. Because the common-mode signals from the conductive pads are the same, a differential amplifier simply cancels out the common-mode and amplifies the input differential EKG signal. The INA332 instrumentation amplifier is a low-cost differential amplifier used in this application and has a common-mode rejection ratio (CMRR) specification of 73 dB up to 10 kHz, quiescent current of 490 μA, and shutdown current levels less than 1 μA. It can operate to a minimum supply voltage of 2.7 V with a dedicated shutdown pin. Additional instrumentation amplifier options relevant to this application are summarized in Heart-Rate and EKG Monitor Using the MSP430FG439.

The INA332 is configured to a gain of 10 V/V with external 0.1% 10-kΩ resistors. The conductive pads are connected to the inputs with 51-kΩ resistors in series to limit the current from the human body and also act as a RC low-pass filter. The 5-MΩ pulldown resistors from the pads to common-mode voltage (VCM) help keep the voltage identical on both inputs and also provide a DC bias point for circuit operation. The VCM voltage is generated by a general-purpose op-amp in the voltage-follower (low output impedance) configuration to 750 mV.

The TLV274 is a quad operational amplifier (op-amp) used in this application with supply currents of 550 μA/channel and minimum supply voltage of 2.7 V. With a CMRR of 58 dB, the op-amps are used to implement a second-order Sallen-Key low-pass filter (LPF) with gain of each stage at 8.5 V/V. The overall gain of the AFE is 10 X 8.5 X 8.5 = 722.5 V/V, and the cutoff frequency is 16 Hz. Additional general-purpose op-amp options relevant to this application are also summarized in Heart-Rate and EKG Monitor Using the MSP430FG439 .

The resulting amplified and conditioned EKG signal output from the LPF is fed to the ADC channel of the MSP430 microcontroller for conversion and processing. The shutdown pin of the INA332 and the VCC pin of the TLV274 are connected to one general-purpose input/output (GPIO) pin of the MSP430 to enable or disable the AFE. The GPIO pin is set to a low state to minimize supply current consumption in sleep mode. As a precautionary measure, the AFE has protection diodes (TPD2E001) on the conductive pads to prevent human electrostatic discharge (ESD) from causing component failure.